There are three major energy systems in the human body that yield ATP (adenosine triphosphate: the body’s energy source). They are the phosphagen system, glycolytic system, and oxidative phosphorylation. The systems will be explained later on in this section. They are important in sport because each system is used differently at different intensities. This is why sport specific training is so important in athletic performance. You must first recognize the difference between anaerobic and aerobic metabolism.

Aerobic metabolism means with oxygen and applies to the oxidative phosphorylation process to yield ATP, while anaerobic (without oxygen) pertains to the phosphagen and glycolytic systems to yield ATP. For example, a marathon runner will use primarily the oxidative phosphorylation system, while a sprinter will use primarily the phosphagen and glycolytic system.

In fact it is not unlikely to see a sprinter hold his/her breath for the entire length of a 100m sprint. The reason being is that for the first 40-60m the sprinter is relying primarily on the phosphagen system. This system tends to produce ATP (energy) at a very high rate and thus the sprinter will use the ATP at a high rate. However, simultaneously the glycolytic system is being used as a secondary resource of energy.

Once the stores of ATP in the phosphagen system are used up, then glycolysis becomes the primary source of energy. This is very interesting because the phosphagen system supplies ATP at a fast rate and the glycolytic system supplies ATP at a slightly slower rate than the phosphagen system. Thus when you watch a 100m sprint event you will notice that the sprinters hit max speed at about 60m, then the Phosphagen system runs low of ATP and the glycolytic system takes over.

Since the glycolytic system produces ATP at a slightly slower rate, you will notice the sprinter start to slow down and decelerate after about 60m. It may appear that the sprinter in the lead is speeding up, when indeed he is just harnessing energy from his phosphagen system longer. Thus, allowing him to stay at max speed while other runners slow down, because the other runners switch over to the glycolytic system which is producing energy at a slower rate.

An athlete’s ability to use his/her energy systems efficiently will have a great influence on the athlete’s performance. This is why sports specific training is so important if athletes are going to be successful. Strength and conditioning specialist can train the athlete to use these systems efficiently and effectively.

If you will notice in the drawing above the colors in the energy systems are two toned. The meaning is that the lighter colors represent a high presence of ATP brought about by that specific energy system. At the beginning of the race there was high ATP production by the phosphagen system, and then the stored and phosphagen produced ATP started to run out (dark color).

At the same time, there was a low ATP yield by the glycolytic system (dark color). Once the ATP ran low in the phosphagen system, the glycolytic system became the higher productive source of ATP. It is at this transition period between the systems in the race, that some exercise physiologist say will depict the winner. Who ever has the largest ATP yield from the phoshagen system can sustain max speed longer and thus pull away from the other runners. The ability of sustaining max speed longer in sports can mean the difference in beating a throw to home plate, catching a touch down pass, first place in a race, or scoring a goal. Therefore, training these energy systems becomes very important.

Watch a 100m sprint event and you will notice at about 60m some of the sprinters will start to decelerate (slower rate of ATP production due to glycolytic system) while one or two others stay at max speed longer (maybe to 65m) and win the race. At that point, it looks as if the sprinters speed up and pull away from the pack, when in fact it is simply the onset of a slower energy system by the other sprinters. You may be wondering, can you train the body to hold max speed longer? Yes, you can train to hold max speed longer and athletes do it all the time.

Phosphagen System

The fastest way to produce ATP is via the donation of a phosphate group to ADP. Now remember ATP is a triphosphate group (3 Phosphates), and an ADP is a diphosphate group (2 phosphates). Therefore, the Phosphagen System simply adds a phosphate group (in the muscle it is known as PC or phosphocreatine) to ADP to form ATP. That may sound confusing but just remember you need 3 phosphates to form ATP, and after the ATP is used it goes back to ADP which is a 2 phosphate group. To reform the ATP in this system, you will just add a phosphate group back to ADP to form ATP as illustrated below.

Once PC is added to ADP it forms one ATP molecule and one creatine molecule. As rapidly as the ATP is broken down to ADP to form energy at the onset of exercise, ATP is reformed via the PC reaction (in box above). However, muscle cells store only small amounts of PC, and thus the total amount of ATP that can be generated by the PC reaction is limited. The combination of stored ATP and PC is called the Phosphagen System.

This system provides energy at the onset of exercise, and during short-term high intensity exercise lasting less than 5 seconds. Refer back to the example above with the sprinter diagram and the contrast of colors. Once the PC stores run out, they require additional ATP to reform, this occurs during recovery. Recovery can take minutes. The ability to replenish depends on the conditioning level and training methods of the athlete.

The Phosphagen System is important in athletics because this is where energy for short-term high intensity exercise comes from. Examples of short term high intensity exercise are, a line backer making a tackle, a basketball player dunking, a baseball player batting, and more. All these activities take only a few seconds to complete and require large amounts of immediate ATP. The Phosphagen System provides a simple fast one enzyme reaction to produce ATP quickly for these short intense activities.

Glycolysis

Glycolysis is the breaking down of glycogen (stored glucose or carbohydrates in the muscle). Glycolysis can rapidly produce ATP with out oxygen. However, it cannot produce ATP as fast as the phosphagen system. Glycolysis involves the breakdown of glucose or glycogen to form two molecules of lactic acid or pyruvic acid (this outcome depends on other metabolic factors). In simple terms glycolysis uses energy from glucose to form ATP. This is the same principle as the phosphagen system but it goes through a much more complex process to form the ATP.

By now you might be thoroughly confused, if you are it is ok. You are probably asking, why is glycolysis important, well have you ever heard of carbohydrates? Sure everyone has heard of carbohydrates, but probably very few people know why we eat them or where they go in the body. You can learn more about carbohydrates in the nutrition section, but first we need to get back to learning glycolysis. For now just consider that glycolysis is essentially the breaking down of carbohydrates into energy.

The drawing above depicts glycolysis. Glucose is broken down by 10 reactions to form pyruvic acid and 2-3 ATP molecules. Notice that there are also 2 hydrogen molecules. These hydrogen’s get transported to another part of the cell called the mitochondria. There the hydrogen’s are used for aerobic metabolism, which we will cover soon. For now we will stick with our sprinting example and anaerobic exercise. In this case, our athlete is producing a large amount of ATP via glycolysis and those two hydrogen’s that are supposed to go into the mitochondria, can’t get in. Remember above I said the hydrogen’s get transported. Well with so much hydrogen being produced the transporters get backed up. The hydrogen’s get taken back to the pyruvic acid which forms lactic acid.

You have just learned why you have to pace yourself. When glycolysis happens at a high rate and the hydrogen transporters can’t keep up, lactic acid gets formed. Lactic acid is one of the primary substances that causes muscle fatigue. If we did not have muscle fatigue, then lactic acid would keep forming in our muscles. As more and more lactic acid is formed, the environment within our muscle cells would become acidic. Eventually the acid would break down the muscle.

Alligators are extremely powerful animals. They are comprised of a very large percentage of fast twitch fibers (glycolytic muscle fibers). Alligators usually kill their prey very fast by clamping down and going into a death role. A death role is a very quick violent move, which requires an extreme amount of energy. Once the prey dies it is put in a safe place, while the gator recuperates from the very brief high intensity activity. They often do not have enough energy to eat after the kill. Therefore, they must rest.

This is because the lactate levels in their muscles go through the roof and their PC stores deplete. Thus, the gators rest until energy stores are back up and the feast will begin. Athletes are the same way. They can only sustain maximal contractions of their muscles for short periods of time before they must rest. There is no way to get around fatigue; it is a wall off physiology that can’t be knocked down.

Now you might be wondering, how does muscle fatigue occur? Well remember in the drawing there are 10 enzymatic reactions in glycolysis. One of those reactions is the rate-limiting enzyme. A rate-limiting enzyme is just what it says; it limits the rate of production. You could think of it as a traffic light. Lactic acid comes in and inhibits this enzyme from working. This slows down glycolysis and thus slows down energy production. Now the athlete has that tight feeling in the muscle. By slowing down glycolysis, the mitochondria can catch up and start allowing the hydrogen’s back in. This reduces the lactic acid production.

Oxidative Phosphoralation

Let’s go into the mitochondria at this point and see what is going on. Aerobic ATP production (Oxidative Phosphoralation) goes on in the mitochondria and involves two metabolic pathways known as the Krebs Cycle and the Electron Transport Chain (ETC). The primary role of the Krebs cycle is to complete the oxidation (hydrogen removal) of carbohydrates, fats, and proteins. Hydrogen removal is important because hydrogen’s contain the potential energy in food molecules.

This energy can be used in the ETC to form ATP. The purpose of oxygen in the body is to accept the hydrogen molecules at the end of the ETC. This is why it is imperative to have oxygen. Without oxygen all these metabolic processes back up and remember we said when the hydrogen’s get backed up they join with pyruvic acid to form lactic acid. Then an accumulation of lactic acid inhibits glycolysis and energy production. However, the body has an amazing ability to get rid of the lactic acid. In fact some of your carbon dioxide production comes from buffering of lactic acid.

You might now have two questions 1) what happens to the hydrogen and oxygen after aerobic metabolism? and 2) why do we breathe out so much more carbon dioxide when we exercise? For question number one, what is H2O, or Water? That’s right two hydrogen’s plus one oxygen equals water. The hydrogen’s and the oxygen’s are joined. The water molecules are used elsewhere. Now where does the CO2 come from? It comes from the Krebs Cycle and lactic acid buffering.

Therefore, when you exercise you speed up the energy systems and thus more CO2 is produced. We could dive much further into aerobic ATP production but we won’t. People have written entire books and have dedicated their life’s work to understanding it. We are just scratching the surface.